We have investigated the relationship between mammary cancer and the presence of specific endogenous and infectious mouse mammary tumor virus (MMTV) genetic elements in high, moderate, and low tumor incidence mouse strains. In the high incidence GR stain, a single endogenous provirus has been identified which segregates with the susceptibility to mammary cancer. A novel MMTV variant has been isolated from the low incidence strain BALB/c that is unique both biochemically and immunologically from all existing MMTVs. Noncoordinate gene expression has also been identified in preneoplastic lesions and mammary tumors of the BALB/c and other mouse strains. A high mammary tumor incidence outbred colony of Mus cervicolor, derived from a feral population, has been established. Interspecies radioimmunoassays and monoclonal antibodies have identified, in this colony, a virus related to, but distinct from, MTVs of laboratory mice. These molecular and immunologic studies are delineating the broad diversity of genes and gene products associated with mammary tumors of a given genus and species.